"Wood" style golf club heads and clubs so equipped are used for long distance driving of a golf ball and may be made of traditional wood or of metal, with brass, stainless steel and aluminum among those metals which have been used. All such clubs are commonly referred to as "woods," notwithstanding the particular material used.
When a golf club is at rest in a normal "address" position (the golfer is said to be "addressing" the ball), the industry standard for the angle between the shaft axis and the ground is in the range of from 55 to 56 degrees in the case of a driver. That angle is called the "lie" angle and is built into the club head. Because the club head sole rests relatively flat on the ground (most contemporary club heads are manufactured with a slightly convex sole) in the address position, the lie angle is also the angle between the shaft axis and an extension of a horizontal tangent to the resting sole surface in the direction of the shaft of the club.
It will be readily understood that decreasing the drag (caused by the velocity of the club head and shaft in a downswing plane) in a wood style golf club would make such a club perform better by providing a higher terminal club head velocity resulting in a corresponding increase in ball impact velocity and thus, a longer drive.
It is also known that when a wood style club is swung in the conventional why the heel of the club becomes the leading edge of the operative airfoil for approximately the first 225 degrees of the total 270 degree (approximately) club downswing. FIGS. 9A, 9B and 9C (all prior art) illustrate a typical wood golf club downswing and clearly illustrate the point. FIGS. 9A, 9B and 9C were derived from time lapse photographs and information published in two magazine articles: "Widen Your Arc," by The Editors of GOLF, December 1986, pp. 22-27, at p. 25, and, "It Isn't Classic But it Works for Fuzzy," by Harvie Ward, GOLF WORLD, June 7, 1985, pp. 50-51. GOLF WORLD is published by Dee Bee Publishing Company, 2100 Powers Ferry Road, Atlanta, Ga. 30339. GOLF WORLD has also published an undated "Famous Golf Swings Collection," which depicts a number of swing sequences of well known professional golfers, similar to the set published on Fuzzy Zeller in the June 7, 1985, issue, and referenced above.
FIG. 9A, is illustrative of a golfer holding a club at a typical downswing starting point (at the peak of the backswing), with club shaft 12 essentially parallel to the ground, and with club face 26 (the ball striking surface) essentially parallel to a downswing plane. (See, also, reference numeral 10A of FIG. 9B.) It is important to note that face 26 remains essentially parallel to the downswing plane when the club is within forty-five degrees from impact with ball 16. See, FIG. 9B, club head position shown at reference numeral 10H. Even at the club head position shown at reference numeral 10J of FIG. 9B, face 26 is still "open" (not square) with respect to ball 16. It is estimated that club head position 10J is about ten degrees from impact with ball 16. It is clear then, that at a point ten degrees before impact, club head 10 is still not quite square with ball 16.
FIG. 9C is included to show the relative position of club head 10 and ball 16 at the moment of impact, where club face 26 is finally, and preferably, fully square to the desired flight path of ball 16. This is the same position as is shown for club head 10K in FlG. 9B.
Most prior art attempts to reduce the effective cross-section of the club have approached the problem presented by drag in the last phase of the downswing; that is, the assumption is made that the club face is the leading edge of the club air foil and streamlining airfoils are designed with that goal in mind. See, for example, U.S. Pat. Nos. 1,555,425 issued to R. McKenzie; 2,550,846 issued to C. S. Milligan; 3,595,577 issued to W. R. Hodge; and 4,444,392, issued to C. R. Duclos; where streamlining is taught to reduce drag and where the assumption is made that the club face is the leading edge of the club head air foil. However, Duclos did recognize that the club head face is not the leading edge of the air foil over much of the downswing. Duclos teaches the use of a slot in the backside of the club head to reduce cross-section (and drag, in the early phases of the downswing) in a plane perpendicular to the face and he teaches the use of an internal cavity connected to that slot to provide:
"air within the cavity . . . in the quick dynamic environment of the club swing [in the final downswing phase] moves as shown . . . into what would otherwise be the base drag area . . . to reduce the base drag of the club head . . . " Col. 3, lines 7-11.
It is clear that Duclos's major theme was reduction of drag during the last few degrees of the downswing during the period of maximum velocity.
Other efforts to reduce drag, using techniques other than streamlining (in particular, reduction of cross-section), also assume that the most destructive drag occurs perpendicular to the club face. For examples of these see, U.S. Pat. Nos. 3,468,544 issued to A. J. Antonious; and 3,997,170 issued to M. B. Goldberg. In the former patent, holes are provided through the club head perpendicular to the face to provide a duct for air flow perpendicular to the club face.
Those prior art designs which provide a recess or depression in the sole area adjacent and parallel to the club face do not teach or suggest that such a recess would be effective in reducing cross-section and, in fact, such recesses as taught do not significantly reduce cross-section or provide streamlining because each of them either teaches a rather small sole trough having a flat surface or teach a sole trough having only a two dimensional curvature. For an example of the latter, see, Braly's U.S. Pat. No. 3,810,631 where such a two dimensional concave sole trough is taught for iron type clubs only.
U.S. Pat. No. 3,976,299, issued to Lawrence et al., suggests that a "wing" configuration with convex top and concave bottom surfaces may be of use in reducing air drag. However, the "foot" located perpendicular to the face of the club head below the "wing" section would appear to be oriented for best efficiency were the face of the club head to be considered the leading edge of the air foil.
The problem with most of these designs is that during the great majority of a classic golf club downswing (beginning approximately 270 degrees prior to ball contact), which begins with the club shaft parallel to the ground and extended behind the golfer's head, the leading edge of the club head airfoil is not the face, but the heel, about ninety degrees removed from the face. See, FIGS. 9A, 9B and 9C. The result is that prior art attempts to reduce drag by optimizing airfoil in a direction perpendicular to the face are ineffective over the major portion of the downswing of the club. Duclos recognized the problem and attacked it with a rear slot and connected cavity configuration.
It is also well known that concentrating the mass in a wood style club head at or behind the point of impact with the golf ball tends to increase horizontal spin on the ball, thus tending to cause hooks and slices; those curvatures in ball flight which are evidenced by the ball curving away to the left or right of the desired flight path. This is believed to be caused by the fact that the club head does not always strike the ball on its "sweet spot." When an offset impact point (offset from that "sweet spot") is encountered and mass is concentrated behind the "sweet spot," a force is created which tends to rotate the club shaft about its axis. That causes the club head to rotate so that the face is not perpendicular to a desired flight path, in turn causing a well known hook or slice. When the mass is concentrated in the extremities of the club head, rather than behind the sweet spot, moment of inertia is increased and there is much more resistance to club head 10 rotation upon impact with ball 16.
Hollow club heads are rather common in metallic "wood" club heads because, if metallic club heads are made solid, they become too heavy for practical use. However, hollow club heads tend to provide a "tinny" sound and feel on impact which does not appeal to some golfers. Furthermore, if the face of a hollow head club is made too thin, it may deform on impact because of the tremendous impact loads. The enclosed top of hollow club heads serves to reenforce the face at an upper edge thereof but the closure adds weight above the center of gravity of the club head which raises the effective center of gravity; an undesirable characteristic. The hosel area of a hollow club is another weak point because of the thin metal in which the hosel is mounted. Metal woods generally have the same shape as traditional wood drivers. It is believed that the majority of metal woods are of the hollow type.